Lift systems these days comprise so-termed double doors, i.e. not only shaft doors, but also cage doors arranged at the lift cage. The opening and closing of the shaft doors is usually carried out by the cage or the cage doors. For reasons of safety, during lift operation all shaft doors must always be closed, with the exception of the shaft door of that story at which the lift cage has just stopped. Similarly, the cage doors must be closed when the lift is not stopped at a story in order to be loaded or unloaded or to be entered or left. For maintenance reasons the shaft doors and/or cage doors can obviously also be opened when the lift cage is located otherwise than in the above-described positions. The state, i.e. the setting, of the shaft doors and cage doors, or the setting of locks by which the shaft doors and/or cage doors are lockable in their closed settings, are monitored with the help of monitoring systems. For this purpose, sensor means, for example in the form of guided devices with safety contact positions, are provided. The safety contact positions are integrated in a series connection in a safety circuit. The arrangement is such that the lift cage can be moved only if the safety circuit and thus all safety contacts integrated therein are closed.
Monitoring systems with safety circuits of this kind are subject to numerous disadvantages, which are briefly listed in the following:                Each safety circuit has inherent problems; including the length of the connections, the voltage drop in the safety circuit and the relatively high assembly cost.        The individual safety contacts are comparatively susceptible to failure; unnecessary emergency stops of the lift system therefore frequently occur.        Notwithstanding a monitoring system with a safety circuit, unsafe or risky situations cannot be avoided; on the one hand the safety contacts individually or in common can be relatively easily bridged over, which is virtually equivalent to the absence of safety precautions, and on the other hand, while an open shaft door does indeed prevent movement of the cage, if the cage is not disposed at the then open shaft door the risk still exists of a fall through the open shaft door.        Intelligent or situation-appropriate reactions, for example in the case of an open safety circuit, are not possible; in particular, it cannot be avoided that persons are unintentionally trapped in the lift cage.        The monitoring system does not permit a specific diagnosis, i.e. in the case of an open safety circuit it can only be established that at least one safety contact and thus one lock or at least one shaft door is open. It cannot be established, however, which safety contact or contacts is or are open.        A precautionary maintenance is not possible, since there are no indications about the state of the safety contacts; it is thus not possible to service the lift system in advance and to replace worn safety contacts in good time, and at a point in time in which the lift system can be shut down without problems, except within the scope of a periodic inspection wherein, however, in many cases taking the lift system out of operation, which is not necessary as such, is carried out.        The availability of the lift is restricted, since an open safety contact always has the consequence of taking the lift system out of operation even if another solution, for example blocking the access region to a non-closable shaft door, would be possible.        
The object of the invention is thus to provide an improved monitoring system of the kind stated in the introduction, by which the disadvantages of the state of the art can be avoided or at least significantly reduced. In particular, the detection of the state of the shaft doors or cage doors by the sensor means shall be able to be analyzed and capable of diagnosis. Moreover, gradual deteriorations of individual subsystems shall be capable of recognition so that preventative maintenance can be initiated in good time.
A further object of the invention is to provide a new monitoring system in which disturbances or failures of individual contacts do not lead to failure of the entire lift system.